Calcium, Magnesium, and Ionic Imbalance

When we talk about ionic imbalance and its relationship to pH, we are talking
about the concentration of magnesium, calcium, boron, and carbonate ions.

Calcium (Ca2+) has a positive charge. Carbonate (CO32-) has a negative
charge. Because of these opposite charges, these ions naturally pair with
one another (similar to two magnets). When this happens with these two particular
ions, the result is calcium carbonate. In a closed reef system, this is
not what one wants. The calcium and carbonate are now bound to one another
making each unavailable, and thus, unusable. Calcium levels drop, which
means corals are being deprived of necessary calcium, and carbonate levels
drop.

So what does this mean for pH? As carbonate is removed from the system,
it is no longer present to act as a buffer. The acid dissociation constant,
pKa, of the system drops closer and closer to the value of water, as more
carbonate is removed. The pH also drops, approaching 7, the pH of pure water.

Why doesn’t this occur in nature?

Magnesium (Mg2+) also has a positive charge, and in the ocean, magnesium
is present in much greater quantities than calcium (approximately 3:1) and
is thus able to preferentially compete with calcium for the bond with carbonates.
This is called ionic competition.

It stands to reason that if there is a higher concentration of magnesium
ions than calcium ions, there is a greater probability of carbonates pairing
with magnesium rather than calcium. For illustrative purposes, imagine a
very crowded room of strangers where you are looking for a few friends;
you are much more likely to bump into strangers than your friends. The same
is true for carbonate and magnesium.

Typically reef aquariums will, in fact, have higher concentrations of magnesium
ions than calcium, but more often than not, they still fall woefully short
of the 3:1 ratio found in nature. This is called ionic imbalance and can
be the result of a number of factors.

So why is it better for carbonates to pair with magnesium? Unlike the nearly
irreversible bond between calcium and carbonates, the bond between magnesium
and carbonates is weaker, thus, they will continually pair and then dissociate,
and both will continue to be available. Calcium and carbonate also pair
and disassociate however the rate at which this occurs is far lower than
the rate for magnesium and carbonate.Their pairing prevents the pairing
of calcium and carbonate; calcium remains available, carbonate remain available
and pH remains stable.

Boron performs a similar function, in that it can pair with calcium to
form a calcium-borate complex, Ca(B(OH)4)2In this case, the borate is screening
the charge on the calcium, and not on the carbonate; the complex formed
is like the magnesium-carbonate complex: easily broken and reformed. So
boron effectively prevents calcium from bonding with carbonate, yet the
calcium remains readily available. This is why we include borate in our
ions™.